Photoelectric converter and radiation reader

ABSTRACT

A radiation reader including a conversion device, and a method of driving the radiation reader. The conversion device includes (a) a pixel including a conversion device for converting an incident radiation into an electric charge, and (b) a reading circuit including at least one field effect transistor for reading the electric charge from the pixel in an amplifying manner as a signal containing a noise. The reading circuit includes first storage means for storing the signal containing the noise, second storage means for storing the noise, and processing means for deriving an output signal not containing the noise based on a first output from the first storage means and a second output from the second storage means.

This is a divisional application of application Ser. No. 10/890,104,filed Jul. 14, 2004, now U.S. Pat. No. 7,005,647 which is a divisionalapplication of application Ser. No. 10/247,331, filed Sep. 20, 2002, nowU.S. Pat. No. 6,787,778, which is a divisional application ofapplication Ser. No. 09/252,073, filed Feb. 18, 1999, now U.S. Pat. No.6,600,160.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a photoelectric converter and aradiation reader, more particularly to a photoelectric converter capableof performing the read operation at higher sensitivity and higher speedand a radiation reader capable of reading the information for radiationrepresented by α-rays, β-rays, γ-rays or x-rays at a high sensitivity.

2. Related Background Art

In the case of a photoelectric converter and a radiation reader forreading the information based on radiation by wavelength-converting theradiation into the sensitivity region of the photoelectric converterwith a wavelength converter such as a photoluminescent body (forexample, scintillator), electric charges based on the input informationphotoelectric-converted by a photoelectric-converting section istransferred to a capacitance to amplify a signal voltage.

However, to amplify a signal voltage by transferring electric chargesfrom the capacitance of a photoelectric-conversion element itself to anexternal capacitance like the case of reading signal charges with aconventional circuit, a relatively-large S/N ratio can be obtained but aparasitic capacitance may be formed on a signal line when arranging aplurality of sensors. For example, when arranging 2,000×2,000 areasensors equivalent to an X-ray film at a size of 200×200 μm per cell andmanufacturing an area sensor with a size of 40×40 cm, a capacitance isformed because the gate and source of a transistor for transferringelectric charges are overlapped each other. Because the above overlapcorresponds to the number of pixels, one signal line has an overlapC_(gs) capacitance of 0.05 pF×2,000 area sensors=100 pF even if one areasensor has a capacitance of approx. 0.05 pF. Because a sensorcapacitance C_(s) is equal to approx. 1 pF, when assuming a signalvoltage generated in a sensor as V₁, the output voltage V₀ of the signalline is obtained from the following expression.V ₀=(C _(s)/(C _(s) +C _(gs)×1000))×V ₁Thus, the output voltage lowers to 1/100.

That is, to constitute an area sensor having a large area, an outputvoltage is greatly lowered.

Moreover, to read a dynamic image under the above condition, asensitivity and a high-speed operation capable of reading 30 images persec. or more are requested. Particularly, in the case of anon-destructive inspection including X-ray diagnosis, higher sensitivityallowing the number of signal charges to increase up to 100 to 400 timesis requested because there is a request for minimizing the dose ofX-rays to be irradiated.

SUMMARY OF THE INVENTION

The present invention is made to solve the above problems and its objectis to provide a photoelectric converter capable of preventing an outputvoltage from lowering due to increase of a parasitic capacitance andresultantly having higher sensitivity and more-advanced performance anda radiation reader having the photoelectric converter.

It is another object of the present invention to provide a photoelectricconverter having a large opening ratio; that is, a large rate of thelight-receiving-section region in an area necessary for one pixel andresultantly capable of achieving a high sensitivity and an advancedperformance and a radiation reader having the photoelectric converter.

It is still another object of the present invention to provide aphotoelectric converter capable of reading a dynamic image.

It is still another object of the present invention to provide aradiation reader capable of further reducing the dose of radiation suchas X-rays.

It is still another object of the present invention to provide aphotoelectric converter having a semiconductor layer comprising in onepixel: a photoelectric conversion element, a reading field-effecttransistor having a gate for receiving signal charges generated in thephotoelectric conversion element and a source and a drain for reading asignal corresponding to the signal charges accumulated in the gate,selection-switch means set between the reading field-effect transistorand a power supply, and reset means for resetting the gate; wherein

the photoelectric conversion element, the reading field-effecttransistor, the selection-switch means, and the reset means are formedon a common insulating support body.

It is still another object of the present invention to provide aradiation reader comprising a photoelectric converter having asemiconductor layer comprising in one pixel: a photoelectric conversionelement, a reading field-effect transistor having a gate for receivingsignal charges generated in the photoelectric conversion element and asource and a drain for reading a signal corresponding to the signalcharges accumulated in the gate, selection-switch means set between thereading field-effect transistor and a power supply, and reset means forresetting the gate; wherein the photoelectric conversion element, thereading field-effect transistor, the selection-switch means, and thereset means are formed on a common insulating support body; and aphotoluminescent body set on the photoelectric conversion element toabsorb radiation and emit light in a wavelength band which can bedetected by the photoelectric conversion element.

It is still another object of the present invention to provide aradiation reader having the following in one pixel: a radiation readingelement which includes an electric-charge discharging layer forabsorbing radiation and discharging electric charges and in which atleast the electric-charge discharging layer is constituted so as to beheld by two conductive layers, a reading field-effect transistor havinga gate for receiving signal charges generated in the radiation readingelement and a source and a drain for reading a signal corresponding tothe signal charges accumulated in the gate, selection-switch means setbetween the reading field-effect transistor and a power supply, andreset means for resetting the gate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1, 10 and 13 are schematic circuit diagrams for explainingone-pixel portion of the photoelectric section of a photoelectricconverter;

FIG. 2 is a schematic plan view for explaining a structure correspondingto the schematic circuit diagram of FIG. 1;

FIG. 3 is a schematic sectional view of the structure of FIG. 2, takenalong the line 3-3 of FIG. 2;

FIG. 4 is a schematic circuit diagram showing a case of arranging two ormore pixels of FIG. 1 like a matrix;

FIGS. 5, 6, 11, and 14 are timing charts for explaining a case ofdriving when applying a radiation reader and a photoelectric converter;

FIGS. 7, 8 and 9 are schematic sectional views for explaining aradiation reader;

FIG. 12 is a schematic circuit diagram showing a circuit structure whenarranging the pixels of FIG. 10 like a matrix;

FIG. 15 is a schematic circuit diagram showing a circuit structure whenarranging the pixels of FIG. 13 like a matrix;

FIG. 16 is a graph showing the relation between ON-resistance, channelwidth (W), and channel length (L) of a transistor;

FIGS. 17, 18, 19 and 20 are schematic sectional views showing one-pixelportions of a radiation reader; and

FIGS. 21 and 22 are illustrations showing X-ray absorbingcharacteristics of a shielding material.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the present invention are described below by referring tothe accompanying drawings. A radiation reader of the present inventionfor reading the information based on radiation is not restricted to anX-ray reader described below but it can be also applied to a reader forreading the information based on α-rays, β-rays, and γ-rays.

First Embodiment

FIG. 1 is a schematic circuit diagram showing one-pixel portion of aphotoelectric converter of the present invention. FIG. 2 is a schematicplan view of the radiation reader shown in FIG. 1 and FIG. 3 shows acase of combining the radiation reader of FIG. 2 with a wavelengthconverting body to use the combination as a radiation reader, which is aschematic sectional view between A and B shown by the schematic planview of FIG. 2.

In each drawing, symbol 21 denotes a photoelectric conversion element,300 denotes a capacitor for accumulating signal charges sent from thephotoelectric conversion element 21, 22 denotes a MOS transistor towhich the photoelectric conversion element 21 and a gate are connected,23 denotes a MOS transistor serving as selection-switch means forselecting each pixel, 24 denotes a MOS transistor serving as reset meansfor resetting the gate of the MOS transistor 22 and the capacitor 300,25 denotes a current source, 26 denotes a MOS transistor for outputtinga signal, 27 denotes a terminal to be connected with a voltage source,28 denotes a reset gate line, 29 denotes a vertical gate line, and 41denotes a voltage source. Moreover, symbol V₂ and V₃ denote signals tobe applied to the gates of the MOS transistors 24 and 23, 42 and 43denote signal sources for outputting the signals V₂ and V₃. When thephotoelectric conversion element 21 has a sufficient capacitance, it isnot always necessary to use the capacitor 300.

As shown in FIG. 1, the photoelectric conversion element 21 is connectedto the gate of the MOS transistor 22 so as to be able to amplify andoutput a signal sent from a photoelectric conversion element. Therefore,it is possible to prevent an output voltage from lowering due toincrease of a parasitic capacitance when arranging a plurality ofphotoelectric conversion elements. Moreover, a photoelectric conversionelement of the present invention is not restricted to a photoelectricconversion element having the same layer structure as a thin-filmtransistor (TFT) described below.

Methods for forming elements are described below in order by referringto FIGS. 2 and 3. In this case, an X-ray-signal reader is described. AMOS transistor is constituted with a TFT.

First, Cr is deposited up to a thickness of approx. 500 Å as a lowermetal layer 2 on a glass substrate 1 serving as an insulating materialthrough sputtering and then, patterned through photolithography to etchunnecessary areas. Thereby, the lower electrode of the photoelectricconversion element 21, the gate electrodes of the MOS transistors (TFT)22 to 24, the lower electrode of the capacitor 300, and lower wirings402 and 412 are formed.

Then, an SiN (silicon nitride) layer 70, an i layer 4, and an n layer 5are deposited in order up to thicknesses of approx. 2,000 Å, 5,000 Å,and 500 Å by the CVD method in the same vacuum state. These layers serveas the insulating layer, photoelectric-conversion semiconductor layer,and hole-injection preventive layer of the photoelectric conversionelement 21, the gate insulating layers, semiconductor layers, and ohmiccontact layers of the TFTs 22 to 24, and the intermediate layer of thecapacitor 300. Moreover, they are used as crossing-portion insulatinglayers of upper and lower wirings. Thicknesses of the layers are notrestricted to the above values. It is possible to most preferably designeach of the thicknesses in accordance with the voltage, current,electric charge, or incoming luminous energy used for a photoelectricconverter. However, a thickness of 500 Å or more not allowing electronsand a hole to pass and capable of functioning as the gate insulatingfilm of a TFT is preferable.

After each layer is deposited, an area serving as a contact hole 408 isetched and then, A1 is deposited up to a thickness of approx. 10,000 Åas the upper metal layer 6 through sputtering or the like. Moreover, bypatterning the A1 film through photolithography and etching unnecessaryareas, the upper electrode of the photoelectric conversion element 21,the source and drain electrodes serving as main electrodes of the TFTs22 to 24, the upper electrode of the capacitor 300, and the upperwirings 406 and 416 are formed. At the same time, the lower wiring 402and the upper wiring 406 are connected to each other in the contact hole408.

Moreover, by etching the n layers of only the channel sections of theTFTs 22 to 24 with a RIE and then, etching the unnecessary SiN layer 70,i layer 4, and n layer 5, each element is separated. Thus, thephotoelectric conversion element 21, TFTs 22 to 24, lower wirings 402and 412, and upper wirings 406 and 416, and contact hole 408 arecompleted.

Furthermore, to improve the durability, the upper side of each elementis normally covered with a passivation film 50 of SiN or the like toform a photoluminescent body layer 51 on the film 50 as a wavelengthconversion body. Incoming X-rays is converted by the photoluminescentbody layer 51 into the light (typically, visible light) having awavelength band which can be converted by the photoelectric conversionelement 21.

As described above, the photoelectric converter is able to form thecommon lower metal layer 2, SiN layer 70, i layer 4, n layer 5, andupper metal layer 6 on which the photoelectric conversion element 21,TFTs 22 to 24, and capacitor 300, and a wiring section 400 aresimultaneously deposited by only etching each layer. Moreover, only oneinjection preventive layer is present in the photoelectric conversionelement 21 and the layers can be formed in the same vacuum state.Furthermore, it is possible to form a gate insulating film and an ilayer which are important for characteristics of a TFT in the samevacuum state. Furthermore, because the intermediate layer of thecapacitor 300 includes an insulating layer not easily causing a leak dueto heat, the capacitor 300 is formed by having preferablecharacteristics. Thus, the photoelectric converter can be manufacturedat a low cost with an advanced performance.

The above forming method can use amorphous silicon. When it is requestedto improve the operation speed of a TFT, however, it is possible toimprove the operation speed by forming an amorphous silicon layer andthereafter, changing the layer to a polysilicon layer through laserannealing.

Second Embodiment

FIG. 4 is a schematic circuit diagram showing a photoelectric converterof the present invention. This embodiment makes it possible to outputsignals through successive scanning by arranging one-pixel portions ofthe photoelectric converter in FIG. 1 like a matrix. Also, thisembodiment is described below in accordance with a case of applying theembodiment as a radiation reader by using a wavelength conversion body.

In FIG. 4, the photoelectric converter (photodiode or the like) 21accumulates electric charges corresponding to the incoming luminousenergy of the light in a photosensitive wavelength region obtainedthrough photosensitive-conversion of X-rays. The photoelectricconversion element 21 is two-dimensionally arranged. Also in this case,when the photoelectric conversion element 21 has a sufficientcapacitance, the capacitor 300 added to each pixel is unnecessary. Oneend of the photoelectric conversion element 21 is connected to the gateof a source-follower input MOS (Metal Oxide Silicon Transistor) 22, thedrain of the source-follower input MOS 22 is connected to the source ofa vertical selection-switch MOS 23 and the source is connected to theload current source 25 through a vertical output line 35, and the drainof the vertical selection-switch MOS (transistor) 23 is connected to thepower-supply terminal 27 through a power-supply line 34 and thesecomponents constitute a source follower circuit.

Moreover, symbol 24 denotes a reset switch, and its source is connectedto the gate of the source-follower input MOS 22 and its drain isconnected to the power-supply terminal 27 through the power-supply line34.

When a signal voltage is generated in the gate of the source-followerinput MOS 22 correspondingly to the electric charges accumulated in thephotoelectric conversion element of each pixel, the above sourcefollower circuit reads the signal voltage with the current amplified.

The gate of the vertical selection-switch MOS 23 is connected to avertical scanning circuit 32 by a vertical gate line 29. The gate of thereset switch 24 is connected to the vertical scanning circuit 32 by areset gate line 28. Moreover, the output signal of the source followercircuit is output to an external unit through the vertical output line35, the horizontal transfer MOS switch 26, a horizontal output line 30,and an output amplifier 31. The gate of the horizontal transfer MOSswitch 26 is connected to a horizontal scanning circuit 33.

Operations of this circuit are described below. First, the circuitresets the photoelectric conversion element 21 by the reset switch 24.Then, the circuit starts accumulation. A signal voltage is generated inthe gate of the source-follower input MOS 22 correspondingly to theaccumulated signal charges. After the accumulation time passes, signalsof pixels selected by the vertical scanning circuit 32 and horizontalscanning circuit 33 are amplified by the source follower circuit andthen, successively output through the output amplifier 31.

The structure of this circuit has advantages that a compact layout isrealized because the power-supply line and reset power-supply line ofthe source follower are used in common and a source-follower outputhaving a high linearity is obtained because the selection switch 23 isset to the power-supply side and thereby, the resistance of theselection switch 23 is absent between the source end and theconstant-current source of the source-follower input MOS 22.

Then, an example of driving of the circuit of FIG. 4 is described belowby referring to FIG. 5.

FIG. 4 is a timing chart showing an example of driving timing. In thecase of this example, X-rays are continuously irradiated. Symbols D₁,D₂, D₃, . . . , and DH denote driving of rows. For example, D₁ showseach timing related to the first row. In D₁, φ₁₁ output from thevertical scanning circuit 32 denotes a reset pulse, φ₂₁ denotes a drivepulse for all lines of one row, and φ₃₁ denotes a read pulse output fromthe horizontal scanning circuit 33. Thereby, the read pulse is sent toan analog-to-digital conversion circuit (A/D) 40 through the outputamplifier 31 and stored in an X memory 41.

The potential of the photoelectric conversion element 21 on one-row lineis reset by a pulse φ_(RESET1) in accordance with the reset pulse φ₁₁,the light due to X-ray irradiation is detected by the photoelectricconversion element 21 for the time of approx. (T₁-T₂) to accumulateelectric charges in a capacitor C₁ 300. A transistor 23 is turned on bya pulse φ_(Drive1) to transfer a potential to each column C₂. Then,potentials are successively output to the A/D from each column inaccordance with a pulse φ_(Read1) through the output amplifier 31.

Thereafter, rows are read from D₂ to D_(N).

The time of T₁ is equal to 33 msec (T₁≅1/30 sec) in the case of, forexample, 30 frames per sec. When photoelectric conversion elements arearranged by 500×500 in rows and columns, it is necessary to read from Dto D500, T₂ becomes almost equal to T₁/500 which is equal to approx. 66μsec, T₃ becomes equal to T₂/500 which is equal to approx. 130 μsec.

These times are determined by the number of frames and the number ofpixels for 1 sec.

For the above driving example, a case is described in which X-rays arecontinuously irradiated. FIG. 6 shows a case of intermittentlyirradiating X-rays like a pulse as the second driving method.

In the case of this example, since lines of all rows of a photoelectricconversion element are simultaneously reset, they are reset by resetpulses φ₁₁, φ₃₁, . . . , φ_(1n). Then, X-rays are irradiated like apulse for a time Tx.

Thereafter, electric charges are successively read in accordance withthe pulses φ₂₁+φ₃₁, . . . , φ_(2n), and φ_(3n) similarly to the case ofthe first driving described for FIG. 5.

In the case of this example, an X-ray-pulse irradiation time Tx isrequired. Therefore, how to determine T₂ and T₃ is slightly differentfrom the case of the above example. For example, when assuming that thenumber of pixels and the number of frames same as the case of the aboveexample are used, T₂ is almost equal to (T₁-T_(x))/500, T₁ is almostequal to 1/30 s, and T₃ is almost equal to T₂/500.

To completely derive the performance of a sensor, a problem may occurthat the influence of a dark current during the accumulation time or thefluctuation of the offset potential of the source follower (SF) of eachcell appears as fixed-pattern noises.

In the first and second driving methods, the first method foreliminating fixed-pattern noises previously stores the output data ofeach element when X-rays are not irradiated in a memory and subtractsthe dark current and the noises due to the offset potential of the SFfrom the output under X-ray irradiation. Thereby, it is possible toimprove the sensor characteristic.

In the second driving method, the second method executes a series ofsequences not performing X-ray irradiation and uses the output when noX-ray irradiation is performed as noise data (N). Moreover, thephotoelectric conversion output when X-ray irradiation is uniformlyperformed is used as (signal+noise) data values (S+N) and the difference((S+N)−N) between the data values is calculated to obtain a signaloutput. Thereby, it is possible to correct noises.

Third Embodiment

FIG. 7 is a schematic sectional view showing the structure of anotherembodiment in which a photoelectric converter of the present inventionis applied to a radiation reader. It is possible to use the circuit ofFIG. 4 as an electric circuit. In FIG. 7, a PIN-type photodiode sensorhas an upper electrode constituted with an ITO 710 and a first Al layer712 serves as a wiring for supplying a negative potential to the ITOelectrode 710. A lower electrode is constituted with a second Al layer708 and the second Al layer 708 is also formed on the upper side ofthin-film transistors (for amplification, selection-switch, and resettransistor) through an insulating layer (SiN layer) 707. Symbol 711denotes an insulating film made of SiN or the like and 709 denotes a PINjunction layer.

The second Al layer 708 serving as the lower electrode of the PIN-typephotodiode sensor is connected with a Cr layer 715 and the Cr layer 715is connected with a gate electrode 702 of the thin-film transistor foramplification (in FIG. 7, connection between the Cr layer 715 and thegate electrode 702 is schematically shown).

The thin-film transistors (for amplification, selection-switch, andreset transistor) are constituted by being superimposed with the Cr gateelectrode 702, an insulating layer 703, a semiconductor layer 704, anohmic layer 705, and a source-drain electrode 706 on a glass substrate701.

Moreover, it is possible to fabricate a capacitor by setting a Crelectrode below the second Al layer 708 through an insulating layer madeof SiN or the like similarly to the structure of FIG. 8.

Because a PIN-type photodiode sensor has a sensitivity in visible light,when using the sensor as a reader for radiation such as X-rays, aphotoluminescent body 714 serving as a wavelength conversion body forconverting X-rays into visible light is set on the upper side of thesensor through an insulating layer 713 made of PI (polyimide, SiO₂, orSiN₄) as shown in FIG. 7.

In the case of the structure of this embodiment, the PIN-type photodiodesensor and respective thin-film transistors (for amplification,selection-switch, and resetting) can use amorphous silicon orpolysilicon.

This embodiment above described makes it possible to obtain thefollowing advantages.

(1) Because a PIN-type photodiode sensor is also set on each thin-filmtransistor (though only an amplification transistor is illustrated inthe form of a sectional view, selection-switch and reset transistors areactually included), it is possible to set a sensor opening ratio to avalue closed to approx. 100% and further increase an S/N ratio.

(2) When constituting the area sensor equivalent to an X-ray film shownin the conventional example by amplifying the signal charges accumulatedby a PIN-type photodiode sensor with the amplifier (source followeramplifier) used for the first embodiment, it is possible to improve S/Ncorrespondingly to the amplification factor of the amplifier.

According to the above Items (1) and (2), it is possible to remarkablyimprove an S/N ratio compared to the case of a conventional area sensor(for example, an opening ratio is improved two times by the structure inItem (1) and charge amplification is improved 100 times by the structurein Item (2), and resultantly the S/N ratio is remarkably improved).

Fourth Embodiment

FIG. 8 is a schematic sectional view showing the structure of anembodiment in which a photoelectric converter of the present inventionis applied to a radiation reader. It is possible to use an embodiedcircuit by applying FIG. 4. In FIG. 8, an X-ray-direct-conversion-typesensor mainly made of PbI₂ constitutes the upper electrode with a firstAl layer 813 and the lower electrode with a second Al layer 810. Bysetting an insulating layer 812 made of PI (polyimide) or the likebetween the first Al layer 813 and PbI₂ 811 of the upper electrode, itis prevented that electric charges are injected into the PbI₂ 811 fromthe first Al layer 813 of the upper electrode. Moreover, in the case ofthis embodiment, it is possible to directly take out the electriccharges due to incoming of X-rays by holding a material such as a-Se1,PbI₂, HgI₂, or Pb0 (in this case, PbI₂ is used) for producing a lot ofelectron-hole pairs in accordance with incoming of X-rays with upper andlower electrodes and applying an electric field between the upper andlower electrodes. The insulating layer 812 shown in FIG. 8 is not alwaysnecessary. As shown in FIG. 9, it is possible to sufficiently take outoutput charges from a sensor not having the insulating layer 812 asoutputs.

A signal-charge accumulating capacitor is constituted with the second Allayer 810 of the lower electrode and the lowest Cr layer 808 toaccumulate signal charges generated due to incoming of X-rays in thecapacitor. Symbol 807 denotes an insulating film made of SiN or thelike.

In this case, the second Al layer 810 of the lower electrode is alsoformed on thin-film transistors (for amplification, selection switch,and resetting) through the insulating layer (SiN layer) 807 as shown inFIG. 7 of the third embodiment.

The second Al layer 810 serving as the upper electrode of theaccumulating capacitor is connected with the gate electrode 802 of theamplifying thin-film transistor, serving as a lowest Cr layer (in FIG.8, connection between the second Al layer 810 and the gate electrode 802is schematically shown).

The thin-film transistors (for amplification, selection-switch, andreset transistors) are constituted by being superimposed with the Crgate electrode 802, an insulting layer 803, a semiconductor layer 804,an ohmic layer 805, and a source-drain electrode 806 on a glasssubstrate 801.

In the case of the structure of this embodiment, the thin-filmtransistor (for amplification, selection-switch, and resetting) can alsouse amorphous silicon or polysilicon similarly to the case of the thirdembodiment.

According to the above embodiments, the following advantages can beobtained.

(1) Because an X-ray-direct-conversion-type sensor is also set on eachthin-film transistor (though only an amplification transistor isillustrated in the form of a sectional view, selection-switch and resettransistors are actually included), it is possible to set a sensoropening ratio to a value closed to approx. 100% and further increase theopening ratio approx. two times.

(2) Because an X-ray-direct-conversion-type sensor mainly made of PbI₂does not require that a photoluminescent body converts X-rays intovisible light, it has a high collection efficiency after convertingincoming X-rays into electric signal charges and resultantly, issuperior in the efficiency of conversion from incoming X-rays intoelectric signal charges.

Moreover, because a direct-conversion material is able to effectivelycollect electric charges in accordance with an electric field, it ispossible to make the thickness relatively thick, increase the X-raycollection value, and raise the quantum efficiency.

(3) By amplifying the signal charges accumulated by anX-ray-direct-conversion-type sensor mainly made of PbI₂ or the like withthe amplifier (sour-follower amplifier) used for the first embodiment,S/N is improved compared to the case of the conventional type(transistor type).

Fifth Embodiment

FIG. 10 shows another example of circuit applicable to the first tofourth embodiments. The circuit shown in FIG. 10 is able to subtract anSF offset or the like from a signal. FIG. 11 shows an example of timingof each of signals φ_(x), φ_(R), φ_(N), and φ_(S). In FIG. 10, symbol<1> denotes a reset random noise generated when resetting a sensoraccumulation terminal section with a reset transistor and <2> denotes anoffset-fixing pattern noise generated in a source follower section.X-rays show X-ray irradiation timing. However, it is also possible tocontinuously irradiate X-rays.

To eliminate the above two types of noises, capacitors C_(T1) and C_(T2)are arranged similarly to the case of the circuit of FIG. 10 to transfersignals accumulated in a dark state to the capacitor C_(T1) inaccordance with the pulse φ_(N) and signals accumulated in a brightstate to the capacitor C_(T2) in accordance with the pulse φ_(s), andinput the signals in the capacitors C_(T2) and C_(T1) to a differentialamplifier in accordance with the pulse φ_(H) and perform subtractionprocessing. Thereby, it is possible to obtain a sensor signal excludingthe above noises <1> and <2>.

FIG. 12 is a circuit diagram showing a photoelectric converter foroutputting a signal through sequential scanning, constituted byarranging the photoelectric converters in FIG. 10 like a matrix.

The one-bit section in FIG. 10 (shown by symbol S in FIG. 12), that is,sensors and amplifiers (amplification transistor, selection transistor,and reset transistor) are arranged up to m sections in X direction and nsections in Y direction and the capacitors C_(T2) and C_(T1) arearranged on each signal line as described for FIG. 10.

Therefore, by alternately reading m×n signal outputs in a dark state anda bright state by shift registers in X and Y directions and applyingsubtraction processing to the outputs by a differential amplifier, it ispossible to obtain signal outputs having less noises and high S/N. Inany case, it is possible to drive pixels arranged like a matrix throughcontinuous or intermittent irradiation with a light source of X-rays orthe like as described above.

Sixth Embodiment

FIG. 13 shows still another example of circuit to be applied to thefirst to fourth embodiments. FIG. 14 shows timings of signals φ_(x),φ_(R1), φ_(R2), φ_(N) and φ_(s). In this case, symbol <1> denotes arandom noise generated when applying reset with a reset transistoraccording to the reset pulse φ_(R1), <2> denotes a 1/f random noisegenerated while electric charges are accumulated in a source followersection, and <3> denotes an offset-fixing pattern noise generated in asource follower section. A sensor cell is reset by turning on the resetpulses φ_(R1) and φ_(R2) and then, accumulation by the sensor isstarted. In the case of an X-ray pulse, X-ray irradiation is startedafter the reset pulse φ_(R1) (pulse <1>) is turned off. It is alsopossible to continuously irradiate X-rays.

To remove the above three types of noises, capacitors C_(T1) and C_(T2)are arranged similarly to the case of the circuit of FIG. 13 to transfersignals accumulated in a dark state to the capacitor C_(T1) inaccordance with the pulse φ_(N) and signals accumulated in a brightstate to the capacitor C_(T2) in accordance with the pulse φ_(H), inputthe signals in the capacitors C_(T2) and C_(T1) to a differentialamplifier in accordance with the pulse φ_(H) and perform subtractionprocessing. Thereby, it is possible to obtain a sensor signal excludingthe above noises <1>, <2>, and <3>.

FIG. 15 is a schematic circuit diagram showing a photoelectric converterfor outputting a signal through sequential scanning by arranging thephotoelectric converters in FIG. 13 like a matrix. In FIG. 15, resetcircuits G₁ and G₂ are built in. Also in the case of FIG. 12, it ispossible to incorporate the reset circuits G₁ and G₂.

The one-bit section in FIG. 13 (shown by symbol S in FIG. 15), that is,sensors and amplifiers (amplification transistor, selection transistor,and reset transistor) are arranged up to m sections in X direction and nsections in Y direction and the capacitors C_(T2) and C_(T1) arearranged on each signal line as described for FIG. 23.

Therefore, by alternately reading m×n signal outputs in a dark state anda bright state by shift registers in X and Y directions and applyingsubtraction processing to the outputs by a differential amplifier, it ispossible to obtain signal outputs having less noises and high S/N. Eachphotoelectric conversion element can be driven by continuous X-rayirradiation and pulses similarly to the case of FIG. 5 or 6.

Because n one-bit circuits are connected to each signal line, noverlapped capacitances C_(gs) of the source and gate of anamplification transistor are connected in parallel. In the case of anX-ray area sensor, n is equal to 500 to 2,000. Therefore, signal-linewiring capacitance C₂ becomes equal to C_(gs)((500 to 2,000). Thus, whenC_(gs) increases, C₂ becomes a very large value.

To read only a static image, increase of C₂ is not a large problem.However, to perform dynamic-image-like read, increase of C₂ greatlyinfluences the read rate.

In the case of FIGS. 2, 3, 7, 8, and 9, a photoelectric conversionsection having a light-receiving section (capacitance to be formedaccording to necessity), reset MOS, and source follower are formed on aninsulating substrate and others are normally constituted with externalcircuits. Therefore, the time constant obtained as the product betweenthe ON-resistance Ron and the capacitance C₂ of a transistor in a sensorcell is the largest problem.

FIG. 16 shows an example of the data for the ratio between theON-resistance, width (W), and channel length (L) of a transistor whenusing amorphous silicon as the material of the transistor. In FIG. 16,the broken line shows the calculated value when the thickness ofamorphous silicon is 3,000 Å and the continuous line shows thecalculated value when the thickness of the amorphous silicon is 1,000 Å.

Symbols ▴, Δ, and • denote measured data values.

Normally, W/L ranges between 2 and 10 and the ON-resistance Ron rangesbetween 1 and 10 mΩ. Because C₂ normally has 16 to 50 pF thoughdepending on design, RonC₂ ranges between 10 and 50 sec. Normally, apulse length (e.g., T₂ in FIG. 5) requires a value three times largerthan RonC2 for read. Therefore, the above pulse length reaches 30 to1,500 μsec. As described for FIG. 5, T₂ is equal to, for example, 66μsec. Though this value can correspond to a switching speed in theminimum region, it may not be able to correspond to the switching speedin the normal range. Therefore, by decreasing the thickness of the ilayer and increasing the W/L to 10 or more, it is possible to correspondto high-speed switching.

In the case of the embodiment shown in FIGS. 2 and 3, the W/L cannot beset to a large-enough value for design of a transistor. However, in thecase of the laminated-type photoelectric converter shown in FIGS. 7, 8,and 9, it is possible to fabricate a transistor by using thesubstantially entire surface of a pixel or photoelectric-element formingregion. Therefore, it is possible to completely reduce the Ron.

In the case of the embodiment in FIGS. 7, 8, and 22, a sufficientswitching speed is obtained and therefore, a structure suitable for adynamic image can be formed.

Seventh Embodiment

In the case of the present invention, it is also possible to form aphotoelectric conversion element and a switching element on asingle-crystal substrate.

FIG. 17 shows a schematic sectional view of a photoelectric converterhaving a photoelectric conversion having a photoelectric conversionsection 1701 and a MOS transistor 1702 on a Si single-crystal substrate510. It is possible to form a photoelectric converter similarly to thecase of forming a photoelectric conversion element and a TFT on thepreviously-described insulating substrate. However, when X-rays passingthrough a photoluminescent body (506) without being completely absorbedare absorbed in single crystal, extra carriers are produced in the Sisingle crystal and become noises against a signal caused by X-raysproduced due to the light emitted from the photoluminescent body.

Therefore, as shown in FIG. 17, it is necessary to electrically separatethe substrate 510 from a sensor or switch region (in FIG. 17, they areelectrically separated from each other in accordance with the P-type andN-type). It is preferable to set the thickness of the P-type region(p-well) 501 to a value two or three times or less the absorptioncoefficient of the luminescent wavelength of the photoluminescent body506 so that the light wavelength-converted by the photoluminescent body506 can be completely detected. The p-well 501 is electrically separatedfrom the substrate 510 by applying a reverse bias. Then, X-rays areabsorbed by only a value equivalent to the thickness (2 to 3 μm or less)of the p-well 501, noises due to direct X-ray absorption are decreased,and performances are improved.

By decreasing the thickness of the p-well (501), X-rays are lessabsorbed and the noise characteristic is improved. Though depending on aphotoluminescent body, when using a Gd-based material as thephotoluminescent body 506, 30 to 50% of X-rays are absorbed by thephotoluminescent 506 and residual X-rays pass through thephotoluminescent 506 without being wavelength-converted by thephotoluminescent 506. However, when absorbing all the X-rays with an Sisubstrate, it is impossible to decide the information obtained thoughthe absorption as a signal or noise. Approximately 1/10,000 X-rays ofapprox. 50 KeV are absorbed by Si single crystal having a thickness of 2nm. Therefore, it is possible to reduce noise components by electricallyseparating from the thickness of the above region.

In the case of the photoelectric converter shown in FIG. 17, it ispreferable to change the threshold voltage of the vertical selectionswitch 23 and that of the reset switch 24. The reason is describedbelow.

First, the input MOS transistor 22 of the source follower of FIG. 1 mustmeet the following conditional expression (1).V _(ds) >V _(gs) −V _(th2)  (1)

In the above expression, ds denotes a potential difference between drainand source, V_(gs) denotes a potential difference between gate andsource, and V_(th2) denotes a threshold voltage.

In this case, it is assumed that the gate voltage when the reset switch24 is turned on is V₂, the gate voltage when the vertical selectionswitch 23 is turned on is V₃, the drain voltage of the input MOStransistor 22 of the source follower is V₁, the threshold voltage of thereset switch 24 is V_(th0), the threshold voltage of the verticalselection switch 23 is V_(th1), and the threshold voltage of the inputMOS transistor 22 of the source follower is V_(th2).

When assuming a case in which both the reset switch 24 and the verticalselection switch 23 operate in a pentode region (region in which thebias (VDS) between source and drain is equal to or lower than apinch-off voltage), the reset voltage V_(sig0) is expressed by thefollowing expression (2).V _(sigo) =V ₂ −V _(th0)  (2)

Then, when assuming that the current flowing through the verticalselection switch 23 is equal to the current flowing through the sourcefollower circuit, the following expression (3) is effectuated.Ia=K(V ₃ −V ₁ −V _(th1))²  (3)K=½×μ×C _(ox) ×W/L

-   -   μ: Mobility    -   C_(ox): Gate oxide film capacity for unit area    -   W: Gate width    -   L: Gate length

In this case, a gradual channel approximation is used to simplify theexplanation.

By transforming the above expression, the following expression (4) isobtained.V ₁ =V ₃ −V _(th1)−√{square root over ((Ia/K))}  (4)

By substituting the expressions (2) and (4) for the expression (1), theconditional expression for the source follower to operate in a linearoperating region is shown by the following expression (5).V ₃ −V _(th1)−√{square root over ((Ia/K))}>V ₂ −V _(th0) −V _(th2)  (5)As an example in which the reset switch 24 and vertical selection switch23 operate in the pentode region, the gate voltages V₂ and V₃ have useda voltage equal to a power-supply voltage and the threshold voltages ofthe switches 23 and 24 have used the same value. In this case, theexpression (5) is transformed into the following expression (6).√{square root over ((Ia/K))}<V _(th)  (6)Thus, it is found that the current which can be flown through the sourcefollower circuit is controlled by the threshold voltage of each switch.

Therefore, when change to multiple pixel is accelerated and the loadwhich must be driven by a source follower circuit increases, it is morepreferable to change threshold voltages of the vertical selection switch23 and reset switch 24 so as to meet the above expressions. For example,it is desired to change the threshold value (V_(th)) of respectivetransistors by on the order of 0.5 V to 1.0 V.

Examples of raising the threshold voltage of the reset switch 24 shownin FIGS. 1 to 3 by 1 V compared to that of the vertical selection switch23 are shown below.

(a) By constituting the gate metal of the reset switch 23 with chromiumand the gate metal of the vertical selection switch 23 with aluminum,the threshold of the reset switch 24 becomes approx. 2.5 V and that ofthe vertical selection switch 23 becomes approx. 1.5 V.

(b) When constituting gate metals of the vertical selection switch 23and reset switch 24 with aluminum, by applying +20 V to the potential V₂of the gate metal of the reset switch 24 in common to all bits andmoreover, grounding the potential V₃ of the gate metal of the verticalselection switch 23 and driving the switches 23 and 24 for approx. 3 hrat ordinary temperature, the threshold of the reset switch 24 becomesapprox. 2.5 V and that of the vertical selection switch 23 is kept atapprox. 1.5 V.

Then, to form a photoelectric conversion element, a field-effecttransistor (MOS transistor) whose gate is connected with the output sideof the photoelectric conversion element on the single-crystal substrate,vertical selection switch, and reset switch, a method for changingthreshold voltages of the vertical selection switch and reset switch isdescribed below.

FIG. 18 is a sectional view showing a method for changing thresholdvoltages. In FIG. 18, symbol 501 denotes a semiconductor substrate, inwhich a P-type semiconductor is shown. Symbol 502 denotes a gateelectrode formed on the semiconductor substrate 501 through a gate oxidefilm, which is formed with, for example, polysilicon or polycide. Symbol503 denotes a source region and a drain region formed in thesemiconductor substrate 501 through ion implantation or the like andhaving a conducting type opposite to that of the semiconductor substrate501. A field-effect transistor is constituted with the above components.Moreover, to improve the durability, the upper side of each element isnormally covered with a passivation film 505 made of SiN or the like toform a photoluminescent layer 506 on the film 505. Incoming X-rays areconverted into the light in a wavelength band which can bephotoelectric-converted (typically, visible light) by a photoelectricconversion element on the photoluminescent layer 506.

In this case, by forming a channel doped layer 504 in the channel regionof only a desired transistor, it is possible to easily make thethreshold voltage of the transistor different from that of othertransistors. For example, in the case of the example of FIG. 18, bydoping the layer 504 with an N-type ion species it is possible to lowerthe threshold voltage of a transistor compared to that of a transistornot doped with the N-type ion species. However, by doping the layer 504with a P-type ion species, it is possible to raise the thresholdvoltage. The change value can be accurately determined by controllingthe concentration of the channel doped layer 504.

An N-type field-effect transistor is described above as an example.However, it is needless to say that the same advantage can be alsoobtained from a P-type field-effect transistor. Moreover, for thisembodiment, a case is described in which the channel doped layer of onetransistor is controlled. However, the present invention is notrestricted to the above case. It is also possible to use a plurality ofchannel doped layers by setting each of them to the optimum condition.

In the above description, the expression (3) of gradual channelapproximation is used as an expression of the current flowing through afield-effect transistor. In addition to the case of the above idealtransistor, even if a slight shift occurs from the above expressionbecause of progress of fining of a transistor, the advantage is notchanged. It is essential to control the on-resistance of a field-effecttransistor so as to meet the expression (1). For this reason, it is veryeffective means to change threshold voltages of a vertical selectionswitch and a reset switch.

The structure shown in FIG. 19 is used for another method for changingthreshold voltages. In FIG. 19, symbol 601 denotes a well regionprovided only for a desired transistor region. Other structures are thesame as those shown in FIG. 18. Also by the structure of FIG. 19, it ispossible to easily control the threshold voltage of a desiredtransistor. Moreover, for FIG. 19, a case is described in which a P-typewell region is formed in a P-type substrate. However, it is alsopossible to determine a desired threshold voltage by forming a pluralityof P-type wells having different concentrations in an N-type substrateand controlling each concentration. Furthermore, though an N-typefield-effect transistor is described as an example, it is needless tosay that the same advantage can be obtained from a P-type field-effecttransistor.

It is also effective to form the direct-type photoelectric conversionelement shown in FIGS. 8 and 9 on a single-crystal substrate. In thiscase, it is preferable that a substrate can be electrically separatedfrom a transistor similarly to the case of FIG. 17. Moreover, it is amatter of course that the circuit and operation previously described canbe applied to a direct-type X-ray sensor.

It is already described that, when using a single-crystal substrate as asensor substrate, transmitted X-rays to be absorbed in the substrate aredecreased. Moreover, it is possible not only to make the region of ap-well or the like thin but also to form a shielding layer at the upperside.

For example, it is preferable to form the gate of a MOS transistor witha heavy metal instead of polysilicon. Specifically, it is preferable toform the electrode 502 of FIG. 17 with a heavy metal (Pt, W, Mo, or Pd).In the case of a MOS transistor, it is preferable to form a metal into adouble or triple structure and moreover, form the lower side withpolysilicon and the upper side with a heavy-metal silicide or a heavymetal.

Furthermore, as shown in FIG. 20, it is possible to form an X-rayshielding layer 600 between the photoluminescent body and the substrateat a portion other than the light detecting section.

FIGS. 21 and 22 show X-ray absorption characteristics of platinum Pt andtungsten W which can be used as shielding materials. For example, byusing a shielding layer made of Pt and having a thickness of 10 μm, itis possible to obtain X-ray shielding effects of 13% and 91% againstX-rays of 50 KeV and 10 KeV. Particularly, the shielding layer is everyeffective for a low energy.

When applying the direct-type structure shown in FIGS. 8 and 9 to asingle-crystal substrate, it is possible to use a shielding layer madeof a heavy metal (e.g. Pt, W, Mo, or Pd), which is the second Al layer810 instead of Al.

As described above, the present invention makes it possible to preventan output voltage from lowering due to increase of a parasiticcapacitance caused when arranging a plurality of photoelectricconversion elements by sending signal charges generated in aphotoelectric conversion element to the gate of a reading field-effecttransistor and amplifying the signal charges with the readingfield-effect transistor to output a signal.

Besides, a photoelectric conversion element may be arranged on at leastany one of the reading field-effect transistor, selection switch means,and reset means, so as to allow the opening ratio to be improved.

Moreover, the present invention makes it possible to provide aphotoelectric converter having higher sensitivity and more-advancedperformance and radiation reader having the photoelectric converter.

1. A radiation reader comprising: a plurality of pixels provided on aninsulating substrate, wherein each one of the plurality of the pixelsincludes a conversion element for convening an incident radiation intoan electric charge, a source-follower input transistor having a gate forreceiving the electric charge from the conversion element and having asource and a drain for reading a signal corresponding to the electriccharge stored in the gate, and a reset transistor for reset of the gate;a circuit for obtaining a signal output based on the signal, wherein thecircuit comprises first storage means for storing the signal containingnoise generated in each one of the plurality of pixels after the resetof the gate by the reset transistor, and second storage means forstoring the noise read by the source-follower input transistor; andprocessing means for subtracting from the signal stored in the firststorage means, the noise stored in the second storage means to obtainthe signal output, wherein the source-follower input transistor and thereset transistor each comprises a thin film transistor formed fromamorphous silicon or polysilicon, and wherein the conversion element hasan upper electrode and a lower electrode, and the lower electrode isformed on an upper side of the source-follower input transistor and thereset transistor through an insulating layer.
 2. The radiation readeraccording to claim 1, wherein the second storage means stores the noiseread by the source-follower input transistor, before the reading of thesignal by the source-follower input transistor.
 3. The radiation readeraccording to claim 1, wherein each one of the plurality of pixelsfurther comprises a selection transistor for selecting the pixel.
 4. Theradiation reader according to claim 1, wherein the conversion elementincludes a wavelength converter for converting the radiation into alight, and a photoelectric converter for converting the light into theelectric charge.
 5. A method of driving a radiation reader having aplurality of pixels provided on an insulating substrate, wherein eachone of the plurality of pixels includes a conversion element forconverting an incident radiation into an electric charge, asource-follower input transistor having a gate for receiving theelectric charge from the conversion element and having a source and adrain for reading a signal corresponding to the electric charge storedin the gate, and a reset transistor for reset of the gate, the methodcomprising: a first step of storing the signal containing noisegenerated in each pixel after the reset of the gate by the resettransistor; a second step of storing the noise read by thesource-follower input transistor; and a third step of subtracting, fromthe signal stored in the first step, the noise stored in the secondstep, to obtain a signal output, wherein the source-follower inputtransistor and the reset transistor each comprises a thin filmtransistor formed from amorphous silicon or polysilicon, and wherein theconversion element has an upper electrode and a lower electrode, and thelower electrode is formed on an upper side of the source-follower inputtransistor and the reset transistor through an insulating layer.
 6. Themethod according to claim 5, wherein the second step is executed afterthe first step.